Method and apparatus for determining the thrust of a jet engine

ABSTRACT

Apparatus is known for determining gross thrust in jet engines by measuring various internal pressures, including measurement of total pressure at the nozzle entrance by means of an immersed probe. The immersed probe has a very short life when an afterburner is used in the engine. The apparatus for the invention is able to determine total pressure at the nozzle entrance, without an immersed probe, from the total pressure in the diffuser and the static pressure at the nozzle entrance. Another embodiment makes use of an additional static pressure in the flameholder region. Also, the invention is for determining the gross thrust of an engine and it distinguishes between an engine with a nozzle operating in a choked condition and an unchoked condition.

United States Patent Murphy et 'al.

METHOD AND APPARATUS FOR DETERMINING THE THRUST OF A JET ENGINE Inventors: John R. B. Murphy, Kanata,

Ontario, Canada; Edelbert G. Plett, Belle Mead, NJ.

Canada Data Canada, Ltd., Ontario, Canada Filed: Jan. 31, 1972 ApplfNo; 222,225

Assignee:

Foreign Application Priority Data Oct. 29, 1971 Canada 126454 References Cited UNITED STATES PATENTS 2/l966 Russ 73/1 17.4

3,258,958 7/l966 Bosch et ul. 73/1174 Primary Examiner-Jerry W. Myracle Attorney, Agent, or FirmWalter C. Farley 5 7 ABSTRACT Apparatus is known for determining gross thrust in jet engines by measuring various internal pressures, including measurement of total pressure at the nozzle entrance by means of an immersed probe. The immersed probe has a very short life when an afterburner is used in the engine. Theapparatus for the invention is able to determine total pressure at the nozzle entrance, without an immersed probe, from the total pressure in the diffuser and the static pressure at the nozzle entrance. Another embodiment makes use of an additional static pressure in the flameholder region. Also, the invention is for determining the gross thrust of an engine and it distinguishes between an engine with a nozzle operating in a choked condition and an unchoked condition.

4 Claims, 5 Drawing Figures I 1 't j l krie kg 1 i IKE; I it 7 gl P 5 ee J2 a j sn j Peeesurze Pxzeosuze 5 '[izmsoucea fi IKANSDUCEJZ. p

Przessuze Pneesumc TR NsoucEn, TwAueoucelz 11 luolcArolc METHOD AND APPARATUS FOR DETERMINING THE THRUST OF A JET ENGINE The invention herein described was developed using funds of the United States and Canadian governments.

This invention relates to thrustmeters and to measurement methods for the determination of jet engine aerodynamic gross (jet) thrust.

The aerodynamic gross thrust of a jet engine is defined as the momentum of the nozzle exhaust gases plus any pressure force which exists due to incomplete expansion of the exhaust gases. This definition, written in equation form, is:

G e/g e( s0 so) wherein:

F aerodynamic gross (jet) thrust m mass flow rate through the nozzle V velocity of exhaust gases g0 dimensional constant A exit area of nozzle P,,,,= static pressure existing at the plane of the nozzle exit and P ambient static pressure.

The previous teachings have developed various functional forms of this fundamental definition equation. It is well known from these teachings that a knowledge of the ambient static pressure and internal pressures in the exhaust region of the jet engine, in addition to a nozzle area, can enable a determination of the gross thrust.

ln the previous teachings the following ratios of pressure are significant:

oz P /P total pressure in the nozzle/ambient static pressure and a P /P total pressure in the nozzle/static pressure at the nozzle entrance. Depending upon the type of nozzle attached to the engine the basic gross thrust equation can be rewritten in the following pressure-dependent function forms: a. complete expansion nozzle (P F ,=P ,,,'g,.(a,.)" f( (2) b. incomplete expansion nozzle (P P F 1gzl( c)' f( e) The difficulties associated with the inflight measurement of the nozzle exit area are overcome in the teachings by trading the A.- measurement for a measurement of the nozzle entrance static pressure P [n this case,

the gross thrust can be computed using the simple equations which follow:

a. complete expansion nozzle F =A,,-P 'a fwd-Ma m. b. incomplete expansion nozzle F. =A,.-P ,,-ar ..'!l94A .15.).

wherein:

F gross thrust P ambient static pressure A area of nozzle entrance f wl.) function of the pressure ratio P /P, for a complete expansion nozzle f,(a,,) function of the pressure ratio P IP for an incomplete expansion nozzle and lz(a-) function of the pressure ratio P IP which is independent .of the type of nozzle employed on the engine.

It will be noted from the above equations that the gross thrust can be calculated from a knowledge of three variables, namely, the nozzle total pressure P the ambient static pressure P and the nozzle entrance static pressure P In this approach the nozzle entrance area A is a constant.

A serious limitation of a current gross thrust measurement technique is the assumption that the nozzle is capable of extending the exhaust gases to the ambient static pressure which exists at the operational altitude of the aircraft. This is a good assumption for low speed sea level operation of an engine fitted with a convergent nozzle. On the other hand for high flight speeds at increased altitudes this assumption, using the same nozzle, will provide a drastic over-prediction of the jet engine gross thrust. It therefore becomes important to recognize when the nozzle of ajet engine is, in fact. op erating so that the exhaust gas is expanded to ambient static pressure, and when the nozzle is not operating so that there is complete expansion. This was not recognized in the past.

A drawback of another gross thrust measuring system is the calibration which is necessary in order to accommodate a specific engine nozzle. ln such a prior art system the well-known thermodynamic function g (1 (or g,(a is replaced by an empirical relationship which is derived from established engine performance data. Changing the type of nozzle will require a recalibration of the tailpipe-nozzle configuration in order to provide an accurate-indication of gross thrust.

A major disadvantage of current gross thrustmeters relates to the measurement technique which enables a calculation of gross thrust for an engine which operates in the afterburning mode. The current systems require a knowledge of the tailpipe (or nozzle) total pressure. This has been obtained using an immersed probe. With the after-burner in operation, special total pressure probes are required because of the extremely hot gases that exist at the measurement station. Attempts have been made to cool the immersed probe by intermittent immersion or continuous cooling, but these have not been successful in an airborne system. An immersed probe will fail in a very short time when the afterburner is operating.

Accordingly, it is a feature of this invention to provide a gross thrustmeter which avoids the use of immersed pressure probes in the tailpipe and nozzle regions of the jet engine.

Another feature of this invention is to provide a gross thrustmeter which can accommodate with minimal, or no, prior calibration both the complete and incomplete expansion nozzles of the fixed or variable area type.

A further feature is to provide an accurate gross thrustmeter which is insensitive to measurement errors associated with the obtaining of representative (average) total and/or static pressures at a measurement station.

Briefly, the present invention in its broadest aspect is for apparatus for determining the total pressure at the nozzle entrance of a jet engine without using an immersed probe. The apparatus comprises a pressure responsive means for detecting the total pressure in the diffuser and providing a first signal representative of said total pressure, pressure responsive means for detecting the static pressure at the nozzle entrance and providing a second signal representative of said static pressure, and computer means for combining at least said first and second signals in a predetermined manner to provide a third signal representing total pressure at the nozzle entrance.

An embodiment of the invention is for apparatus for determining gross thrust in a jet type engine, including apparatus which determines the total pressure at the nozzle entrance, and further comprises means for combining signals representing the total pressure at the nozzle entrance, ambient static pressure, predetermined constants and conditions to select one of two predetermined relationships and evaluate it to obtain an indication of gross thrust.

The invention will be described hereinafter in greater detail with reference to the accompanying drawings, in which FIG. 1 is a schematic diagram of the components of the invention,

FIG. 2 shows diagrammatically basic types of nozzles which are fitted to jet engines,

FIG. 3 is a graph showing the variation of the functions fli r) and fr( r-) t TA/ m) FIG. 4 is a block diagram showing a gross thrust system, and

FIG. 5 is a block diagram showing a modified gross thrust system.

Referring to FIG. I of the drawing, there is shown in schematic form, an aftcrburning jet engine with a diffuser l, tailpipe (afterburner) 2 and nozzle 3. A computer 4 which receives signals from four pressure transducers ll, l2, l3 and 14 responsive, respectively, to

total pressure (P from diffuser section at probe 7,

static pressure (P from the flameholder area at tap 8, nozzle entry static pressure (P at tap 9, and altitude ambient static pressure (P,,,) which is indicated as available at detector 10. It should be noted that the pressure responsive device at 8 for detecting P can be located at any convenient place between the upstream side of the diffuser and the nozzle entrance region where the device for detecting P is located at 9. In an engine with an afterburner, as shown, it is conveniently located in the afterburner region or flameholder region. in an engine without an afterburner it could be located in the diffuser. It should also be noted that for airborne installations a signal which is indicative ofthe altitude ambient static pressure is already available from the aircrafts flight instrumentation or control system. The computer 4 produces a resultant signal, which is indicative ofgross thrust. This signal is fed to an indieating instrument 5 which displays the aerodynamic gross thrust F It will be apparent that two pressures may be detected and a signal representing the sum or difference applied to computer 4.

FIG. 2 shows examples of the two nozzle types referred to in the discussion of equations (10) and (l l FIG. 3 is included to show generally, by way of example, the difference between two functions involved in determination of gross thrust for a complete expansion nozzle and an incomplete expansion nozzle as discussed in connection with equations l0) and (11) and as previously referred to. It will be seen that as a or P /P increases the two functions f w.) and f,(a,.) become further apart. The prior art did not recognize this and consequently there were inaccuracies in determination of gross thrust.

In the gross thrustmeter according to the invention it is not necessary to use a total pressure probe which is immersed in the gas stream in the tailpipe or nozzle re gions. Thus, the jet engine can operate in the afterburning mode or not without affecting the determination of gross thrust. lnternal engine pressures and the altitude ambient static pressure are the only dynamic parameters sensed in order to provide an indication of the jet engine gross thrust. Total pressures are sensed using conventional total pressure probes installed in the low temperature environment of the engine, i.e., upstream of the flameholder. Static pressures are available using static wall taps located in the engine tailpipe downstream of the flameholder. The exact location of the wall tap is not significant. The pressures need not be representative of the measurement station pressures since a simple static sea level calibration of the engine provides the means for correcting the measurements. The pressures may have to be corrected to account for radial pressure gradients which exist due to the threedimensional character of the engine flow. A signal representing the altitude ambient static pressure is already available in the aircraft's flight instrumentation system. Thus, all the signals are readily available and serve as inputs to a computer which solves novel equations to determine gross thrust.

It will be recalled that it was previously indicated that the prior art did not recnognize a difference between an engine operating so that there was complete expansion of the exhaust gases and so that there was incomplete expansion. It has been discovered that certain functionsf,(a,.) andf (u,.) appearing in equations used in determining gross thrust vary in a slightly different manner with a... This is shown in H6. 3 and will be referred to in more detail hereinafter.

In the present invention, a novel equation based on sound acre-thermodynamic principles, is employed to calculate the nozzle total pressure P using the diffuser total pressure P and at least two independent tailpipe static pressures. Depending upon the geometry of the nozzle as well as the sensed pressures, the computer selects one of the following equations to compute the gross thrust: complete expansion nozzle: F A -P,,,-a,.f (a )-h(a incomplete expansion nozzle: F A-P, -a,.-f,(- at Km)- Thus, the present invention involves implementation of the novel general theoretical relationship to determine the nozzle total pressure without actually measuring it. Furthermore, implementation of both gross thrust equations is carried out for selective use in determining jet engine aerodynamic gross (jet) thrust.

A convergent nozzle can expand engine exhaust gases to the altitude ambient static pressure whenever the Mach number at the nozzle exit plane is less than unity. This normally occurs at lower power settings at low altitudes and would be expected, for example, at take off. A variable exit area convergent-divergent nozzle can be constructed to essentially accomplish the complete expansion regardless of flight altitude and engine throttle setting. Thus, the static pressure at the nozzle exit plane P will be greater than the ambient static pressure P whenever a convergent nozzle commences )peration at nozzle pressure ratios 'rx/ (TH/2) Y Y (turbojet engine: single exh 1st stream) Warm P nozzle total pressure P ambient static pressure and 'y exhaust gas ratio of specific heats.

It is desirable to know the total pressure at the nozzle (P without using an immersed pressure probe. ltwill be recalled the exhaust gases are extremely hot at the nozzle when the engine is operating in the afterburning mode.

The following thermodynamically-exact equation relates P and P using two independent engine static pressures, predetermined functions of the pressures or differences thereof, known constants and predetermined functions of these constants:

cipline that the method of enabling the obtaining of an indication of nozzle total pressure P does not depend upon any preferred location of the pressure probes (and taps). Consider, for example, a simple afterburning turbojet (compressor, combustion burner, turbine, diffuser, tailpipe and nozzle) engine. The conventional total pressure probe may be located anywhere in the diffuser section, i.e., downstream of the final turbine stage up to the tailpipe flameholder. The nozzle static pressure tap which indicates P must be located downstream of the flameholder. The static pressure P (defined as the flameholder static pressure) may be measured using a wall tap located downstream of the flameholder but upstream of the wall tap which measures P Either a wall tap or' an immersed static pressure probe situated downstream of the final turbine stage, but upstream of the flameholder, will also provide the necessary indication representing P It has been discovered to be possible to eliminate one independent static pressure measurement from the present gross thrustmeter system if the engine is tested, for calibration purposes, in both the dry (nonafterburning) and afterburning modes of operation. The present invention is therefore also concerned with the mechanization of the following simple technique which enables a calculation of the nozzle total pressure using two independent internal engine pressures and a few constants.

The relationship between P P -/P and P /P is essentially a straight line for both the afterburning and non-afterburning modes of engine operation. This fact is displayed by the following simplee quations:

a. afterburner on: P P (I a (P,,- -/Pr5) b )(8) K dimensionless constant K dimensionless constant derived from the tailpipe geometry K 3 dimensionless constant derived from the known thermodynamic properties of the engine exhaust gas P, nozzle entry static pressure P static pressure near the flameholder 7 I'm/P total pressure upstream of the flameholder/static pressure near the flameholder f (a =,),f (a functions of the pressure ratio P lP wherein:

P total pressure measured upstream of the flameholder P nozzle total pressure P static'pressure measured downstream of the flameholder a, slope of the straight line representing an afterburning characteristic a slope of the straight line representing a nonafterburning characteristic b intercept of the afterburning curve on the axis used to represent P P IP and b intercept of the non-afterburning curve on the axis used to represent P P /P The nozzle total pressure P may therefore be calculated using the internal engine pressures P and P in one of the equations listed above. The equation selected is determined by the value of the ratio PsN/P In accordance with the present invention, the operation ofthe thrustmeter is based on the following considerations:

l. The nozzle total pressure P may be calculated using exact (7) or empirical (8 or 9) equations. Both equations are independent of the nozzle type fitted to the engine. The use of the exact equation necessitates the sensing of at least three independent internal engine pressures and requires an initial sea level static test of the engine running in the non-afterburning mode only. Employment of the empirical equations requires a measurement of at least two independent internal engine pressures. However, the engine must be initially static-tested at sea level over the entire range of throttle setting.

2. The aerodynamic gross thrust of a convergentdivergent nozzle may be computed using the complete expansion gross thrust equation:

F6 aa e fc( e)' N) wherein:

F G gross thrust A area of nozzle entrance P ambient static pressure a P /P, nozzle pressure ratio s ry/ nozzle total pressure/nozzle entry static pressure f (a,,) function of the nozzle pressure ratio P /P for a complete expansion nozzle and Ma function of the pressure ratio P y/Psy The ambient static pressure P and nozzle entry static pressure P are measured; the nozzle total pressure P is calculated as per (i) above; the area of the nozzle entrance Ay is known and the functions f (a,.) and Max) are predetermined.

3. The aerodynamic gross thrust ofa convergent nozzle may be computed using either the complete expansion gross thrust equation (equation above) or the incomplete expansion gross thrust equation which follows:

F A P a,., 01 and I1(a are defined in (2) above andf,(a..) predetermined function ofthe nozzle pressure ratio P -/P,,, for an incomplete expansion nozzle. The value of the engine nozzle pressure ratio Pry/P (see equation 6) dictates which equation is employed to compute the gross thrust. If Pry/P3,, ('y+l/2) v either the complete or incomplete expansion equations may be used. The complete expansion equation is necessarily employed whenever Pry/P becomes less than ('y+l/2) Y on the other hand, a choked convergent nozzle operation, (i.e., incomplete expansion) is detected by values of P y/P which are greater than ('y+l/2) v 1 and use is therefore made of the incomplete expansion equation.

Referring now to FIG. 4. there is shown in block form an example of circuitry suitable for a gross thrust meter according to this invention. Signals are provided from pressure transducers 11, 12, I3 and 14 and these signals represent, respectively, total pressure P from the diffuser, static pressure P from the flameholder area, static pressure P from the nozzle entry region, and ambient static pressure P These signals are applied to a multiplexer 16 which provides as its output a series of signals variable in accordance with the signals applied to it. These signals are applied to an analog to digital converter 17 which converts the signals to digital form. The digital data is applied to an arithmetic unit 18. Associated with arithmetic unit 18 are three memory units. A short term memory 19 or scratch pad memory. a constant memory 20 and a main program memory 21. The short term memory 19 scales the input data and stores it temporarily for use by the arithmetic unit 18 and as required by the-program memory 21. The constants memory 20 receives, in digital form, signals representing constants involved in equations (7 l l and also a representation that the nozzle is nonconvergent (i.e., convergent-divergent) or is convergent only.

It will be recalled that there must be a determination of P or total pressure at the nozzle. The necessary values are present in memories 19 and 20 to solve equation (7). It is believed this is well within the ordinary skill ofa workman in the computer arts. The block indicated as 22 in the program memory 21 solves equation (7) for P and this is applied to circuitry represented by block 23 which provides a signal representing a i.e., representing P /P As was previously explained it is necessary to ascertain (a) if the nozzle is convergent or convergentdivergent, and, depending on this, (b) if there is complete or incomplete expansion of exhaust gases. The decision circuitry receives over 25 a signal (originating in the constants memory 19) that the nozzle is or is not convergent. If the nozzle is not convergent, (i.e., if it is convergent-divergent) the complete expansion equation (equation 10) is selected as is indicated by block 26. If the nozzle is convergent a signal representing 01,. is applied to decision circuitry represented by block 27 which determines whether there is complete or incomplete expansion. When a, ('y-l-l/Z) v Y the expansion is considered as complete and equation (10) is selected as in block 26, otherwise the incomplete expansion equation (equation I l) is used as indicated by block 28.

The appropriate equation is available via an interface 30 and is solved by arithmetic unit 18 for gross thrust. The signal representing gross thrust is applied over conductor 31 to an indicator interface 33 which also receives a control signal on conductor 32. The indicator drive signal is applied to indicating instrument 5 over conductor 34.

It is believed that the computer circuitry for solving the equations set forth is within the ordinary skill of a computer technician and that no further description is required.

It was previously stated that when using the empirical equations (8) and (9), only two independent internal engine pressures are required. This embodiment is shown in FIG. 5.

Referring to FIG. 5 it will be seen that the circuitry is quite similar to that of FIG. 4. In FIG. 5 there is no flameholder static pressure transducer, and the circuitry indicated within the main program memory 21 is altered. Thus, in FIG. 5 the two transducers I1 and 13 provide signals representing P and Psy while the flight instrumentation 15 provides a signal representing ambient static pressure P These signals are multiplexed by multiplexer 16 and converted to digital form by analog to digital converter 17. The digital data from converter 17 is applied to arithmetic unit 18 as are the constants data from memory 20.

The program memory 21 contains a decision circuit 40 which receives over a connector indicated as 41 a data signal, which conveniently may be an input to memory 20, representing the afterburner operation. As was described in connection with equations (8) and (9), the circuit 40 selects the equation represented by either block 42 or 43. The circuitry in the selected block provides as an output a signal representing P and this is applied to the circuitry represented by block 23 which determines 01,. The remaining circuitry in the nozzle entrance as represented by said fourth program memory 21 operates in a manner similar to signal, that described in connection with FIG. 4 and similar K is a dimensionless constant designation numbers are used. Similarly the manner in K is a dimensionless constant derived from tailpipe which the indicating device is driven is the same in 5 geometry the FIG. 5 embodiment as in the FIG. 4 embodiment. K is a dimensionless constant derived from the It is believed that no further description is necessary for known thermodynamic properties of the engine exan understanding of FIG. 5. haust gas it will be apparent that the invention described proa P st; vides an improved thrustmeter of novel design which fi,(a,=,) and f (a are functions of the pressure ratio avoids the use of immersed probes and which can ac- P/PS6 commodate different nozzles and different operational fl( S6v S.\)s f (P ,,,P,,- and f (P ,P are functions modes with minimal calibration. of the static p r -PS6 n Psi, an

The embodiments of the invention in which an exclufid z) and f4( 2) are functions of h Constant 2 i propel-[y or i il i l i d are d fi d as f 15 2. A gross thrust meter for use in combination with Iows: a jet type engine which includes a diffuser and a nozzle having an entrance and an exhaust, said thrust meter comprising pressure responsive means for detecting total presl. Apparatus for use in combination with a jet type engine having a diffuser and a nozzle, for determining the total pressure at the entrance to the nozzle, comprising pressure responsive means for detecting the total pressure in the diffuser and providing a first signal signal representing this pressure; pressure responsive means for detecting only one pressure at the nozzle entrance, said one pressure representative 9 Said tom] Pressureg being static pressure P at the nozzle entrance,

this pressure; sefond Signal @p f sald 5mm Pressure pressure responsive means for providing a third sigp i pf f t g gi f p g fh fi the nal representing a pressure P which is the static P ream e 0 e user 6 pressure at a position within said engine in the trance and upstream of said pressure responsive fl h ld i means for detecting the static pressure at said nozmeans f producing an analog i l e ntin zle entrance, for detecting static pressure and probi Static pressureviding a fourth signal representative of said static means fo producing a signal indicative f whether pressure, said nozzle is convergent or convergent-divergent; Q P" a ij Said first and ffj means for producing a signal iidicfative of the choked sIgna s in a pre etermme manner to provi e a or h k d erating me e 0 said engine; t d Sig r pr n ng will] pressure the an analog to digital converter for receiving said first, zle BH I C second and third signals, and said analog signal repsaid computer means being constructed to combine resenting ambient static press r nd for said fourth signal with said first and second signals 40 viding output data including data representing the in a predetermined manner to provide said third pressures P P and P signal according to the relationship computer means for receiving said output data and s sN s( ss, siv) ums) [faUQ) -[f1( 5) K11 P :P f2( ss, siv) fi( s6 sN) TN SN f4(K2) fs( s6, siv) f z( ss, sN)

wherein digital data representing predetermined constants and values associated with engine operation, and providing an output representative of gross thrust, said computer means including logic for evaluating P is the total pressure at the nozzle entrance represented said third signal,

m i f totfll Pressure in the diffuser represented by input data to provide P the total pressure at the Said first 512mb nozzle entrance, including means for combining P, is the static pressure at the nozzle entrance repre- P P and P according to the following relationsented by said second signal, ship:

fl( S6s SN) fs( ss, s1v) P is the static pressure at position within said enwherein gine between the upstream side of the diffuser and K is a dimensionless constant sure P in the diffuser and providing a first analog K is a dimensionless constant derived from tailpipe geometry K is a dimensionless constant derived from the known thermodynamic properties of the engine exhaust gas f (a and f (a are functions of the pressure ratio PTS/PSG f1( ss, sN) f2( ss s-) and fs( sm s.v) are functions of the static pressures P and P and fa(K2) and f4(K are functions of the constant K said computer means including logic for evaluating a where a P /P said computer means including first and second decision logic means, and first and second output logic means,

said first decision logic means being responsive to a signal indicative of said nozzle being convergent-divergent for selecting said first logic output means, and responsive to a signal indicative of said nozzle being convergent only for selecting said second decision means,

said second decision means being responsive to sig nals indicative of unchoked nozzle operation for selecting said first output logic means; and responsive to signals indicative of choked nozzle operation for selecting said second output logic means.

3. A method for determining the total pressure at the entrance to the nozzle of a jet engine without using an immersed probe comprising detecting the total pressure P in the diffuser of said engine;

detecting the static pressure P at the nozzle entrance;

detecting the static pressure P at a position within the engine between the upstream side of the diffuser and the nozzle entrance;

producing signals representing the detected pressures and combining at least these signals in a predetermined manner to obtain a signal proportional to the total pressure P at the nozzle entrance, in accordance with the relationship whcreinf f andf are predetermined functions of the pressures detected.

4. Apparatus for use in combination with a jet engine for determining the total pressure at the entrance to the nozzle of the engine without using an immersed probe comprising means for detecting the total pressure P in the diffuser of said engine;

means for detecting the static pressure P at the nozzle entrance;

means for detecting the satic pressure P at a position within the engine between the upstream side of the diffuser and the nozzle entrance;

means for producing signals representing the detected pressures and combining at least these signals in a predetermined manner to obtain a signal proportional to the total pressure P at the nozzle entrance, in accordance with the relationship P UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,835,701 Dated September 17, 1974 Inv n fl John R. B. Murnhv et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 48:

Change "P o-g I(a. H to S0 g (fi (A Column 2, line 28:

Change "extending" to -expanding--.

Column 5, line 17:

Change P /P 7 (v l/2)v/ 1 Y Y-l to "P /P -:((v 1)/2) line 52:

Change "'to -a Column 6, lines 29, 55 and 58:

Change "P P /P to (P P )/P (each occurrence).

(OVER) FORM PC4050 (10-69) USCOMM-Dc 5031 4559 a 0.5. GOVIINIINI PRINTING orrlcl nu o-su-su,

I C UNITED STATES PATENT OFFICE -CERTIFICATE OF CORRECTION Patent No. 3,835,701 Dated 7 September 17, 1974 Inventor) John R B. Murphy and Edelbert G. Plett It is certified that error appears in the above-identified patent and that saidtLetters Patent are hereby corrected as shown below:

Change the assignee from "Canada Data Canada, Ltd." to

, -Control Data Canada, Ltd.--.

Signed and sealed this 17th day of December 1974. v

(SEAL) Attest:

McCOY M. GIBSON JR. 0. MARSHALL DANN Arresting Officer Commissioner of Patents FORM po'wso USCOMM-DC scan-Pas I I I 0.5. GOVERNMENT PRINTING OFFICE C 0-3..8, 

1. Apparatus for use in combination with a jet type engine having a diffuser and a nozzle, for determining the total pressure at the entrance to the nozzle, comprising pressure responsive means for detecting the total pressure in the diffuser and providing a first signal representative of said total pressure, pressure responsive means for detecting the static pressure at the nozzle entrance and providing a second signal representative of said static pressure, pressure responsive means, positioned between the upstream side of the diffuser and the nozzle entrance and upstream of said pressure responsive means for detecting the static pressure at said nozzle entrance, for detecting static pressure and providing a fourth signal representative of said static pressure, computer means for combining said first and second signals in a predetermined manner to provide a third signal representing total pressure at the nozzle entrance, said computer means being constructed to combine said fourth signal with said first and second signals in a predetermined manner to provide said third signal according to the relationship
 2. A gross thrust meter for use in combination with a jet type engine which includes a diffuser and a nozzle having an entrance and an exhaust, said thrust meter comprising pressure responsive means for detecting total pressure PT5 in the diffuser and providing a first analog signal representing this pressure; pressure responsive means for detecting only one pressure at the nozzle entrance, said one pressure being static pressure PSN at the nozzle entrance, and providing a second analog signal representing this pressure; pressure responsive means for providing a third signal representing a pressure P6 which is the static pressure at a position within said engine in the flameholder region; means for producing an analog signal representing ambient static pressure; means for producing a signal indicative of whether said nozzle is convergent or convergent-divergent; means for producing a signal indicative of the choked or unchoked operating mode of said engine; an analog to digital converter for receiving said first, second and third signals, and said analog signal representing ambient static pressure Pso, and for providing output data including data representing the pressures PT5, PSN and Pso, computer means for receiving said output data and digital data representing predetermined constants and values associated with engine operation, and providing an output representative of gross thrust, said computer means including logic for evaluating input data to provide PTN, the total pressure at the nozzle entrance, including means for combining PSN, PT5 and PS6 according to the following relationship:
 3. A method for determining the total pressure at the entrance to the nozzle of a jet engine without using an immersed probe comprising detecting the total pressure PT5 in the diffuser of said engine; detecting the static pressure PSN at the nozzle entrance; detecting the static pressure PS6 at a position within the engine between the upstream side of the diffuser and the nozzle entrance; producing signals representing the detected pressures and combining at least these signals in a predetermined manner to obtain a signal proportional to the total pressure PTN at the nozzle entrance, in accordance with the relationship
 4. Apparatus for use in combination with a jet engine for determining the total pressure at the entrance to the nozzle of the engine without using an immersed probe comprising means for detecting the total pressure PT5 in the diffuser of said engine; means for detecting the static pressure PSN at the nozzle entrance; means for detecting the satic pressure PS6 at a position within the engine between the upstream side of the diffuser and the nozzle entrance; means for producing signals representing the detected pressures and combining at least these signals in a predetermined manner to obtain a signal proportional to the total pressure PTN at the nozzle entrance, in accordance with the relationship PTN f(PSN, PS6, and PT5). 